When comparing iMessage vs Discord, the Slant community recommends Discord for most people. In the question“What are the best Skype alternatives?”Discord is ranked 3rd while iMessage is ranked 12th. The most important reason people chose Discord is:

Discord follows the same type of interface design popularised by Slack, which is extremely clean and attractive, and doesn't clutter the interface with unnecessary chrome and cruft.

Pros

Pro

Comes pre-installed

Pro

Clean UI

UI is minimalistic, and clear. And there is a plethora of stuff on the left of text box if you wanna use them.

Pro

Cross-platform within Apple ecosystem

iMessage is available on iPhone, iPod, iPad, and Mac.

Pro

Friends don't need to "sign up"

You can just message them using their existing phone number or Apple ID and they will get your message.

Pro

Free and unlimited messages over Wi-Fi and data

Messages clearly indicates whether messages are being sent over iMessage (blue) or SMS (green) where messaging rates apply. iMessages do not count as SMS messages with your carrier, instead they use a negligible amount of data to send over Wi-Fi and cellular data networks.

Pro

Supports voice messages

Short voice recordings/messages can be quickly sent, and messages can be easily dictated into written messages as well.

Pro

Clear indicators for message delivery and receipt

The app indicates when messages have been sent, when they have been read, and when someone is typing.

Pro

Secure end-to-end encryption

Pro

Seamlesss switching between SMS messages

Pro

User friendly

Discord follows the same type of interface design popularised by Slack, which is extremely clean and attractive, and doesn't clutter the interface with unnecessary chrome and cruft.

Pro

Can be used in browser

Users can invite others to join a Discord channel via a web link, which means no installation is required to use the software - which is one of the big typical barriers to entry for social software.

Pro

Free for unlimited users

Unlimited amount of users.

Pro

Easy setup

You can quickly send invites to people and they can join in using the web client without having to create an account or install the app.

Pro

Good 1-click invite system

Discord allows inviting people to a specific channel with a link. Links can be set to expire. Low effort/passive invite mechanics.

Pro

Open source bots and management tools

Discord has a wide variety of bots for things from airhorn noises to automated server management, to leveling systems for chat. The bot API is open and there are dedicated sites for finding bots.

Pro

Game integration

Discord shows which game each user is currently playing, which among other things helps users to form PUGs with minimal hassle.

Pro

Has low-latency VoIP support

Pro

Supports video and text communication

Behaves and looks like Slack for regular text channels, but with the ability to create voice channels as well

Pro

Familiar for Slack users

For those that are used to the way Slack (a similar chat client) works, Discord will be familiar in its use, which lowers the learning curve quite a bit for those that have switched or tried Slack before.

Pro

Android app has push notifications

Pro

DDoS mitigation

Discord prevents users from being DDoSed out of games.

Pro

Low effort invite mechanics

Pro

Syntax highlighting and Markdown support

The support is limited but still exists. It's a killer feature for dev teams.